As power amplifiers for mobile communication in recent years, MMICs (Microwave Monolithic Integrated Circuits), hybrid ICs, and multichip modules, including GaAs FETs (Field Effect Transistors) or AlGaAs/GaAs HBTs, have generally been employed. It is to be noted that an AlGaAs/GaAs HBT is an HBT having an AlGaAs active layer on a GaAs substrate.
Particularly an AlGaAs/GaAs HBT is expected as a power element for future mobile communication because it has the following advantages over the conventional FET.
(1) An AlGaAs/GaAs HBT does not require a negative gate bias voltage, i.e., it is operated with a single power supply.
(2) The HBT is able to perform ON/OFF operation without using a drain switch like an Si-MOSFET.
(3) Since the power density of the HBT is high, the same output power is obtained with reduced chip size as compared with an FET power amplifier.
However, in contrast with an FET, an HBT is operated by a base current applied thereto, and a base current of several tens to several hundreds of milliampere (mA) is required for an output power of 2.about.4W. Hence, in a power amplifier having an HBT element in an amplifier stage, such a base current must be applied to the HBT element. However, it is difficult to obtain such a current from a reference voltage supply unit, such as an LSI comprising a standard Si-CMOS, disposed in the previous stage of the power amplifier, because, in a standard Si-CMOS, an output current value that secures a specific output voltage is lower than 1 mA. Accordingly, in the power amplifier having an HBT element in the amplifier stage, the structure of a bias current for supplying a bias current to the amplifier stage is important.
Especially in portable telephone systems, such as Europe GSMC (Global Special of Mobile Communication) or CDMA (Code Division Multiple Access) systems, for which use of a power amplifier having an HBT element in an amplifier stage is expected, it is necessary to reduce the current level in idle time of the power amplifier for long life of batteries.
FIG. 6 shows input-output characteristics of the power amplifier having an HBT in the amplifier stage. In FIG. 6, P.sub.in is the input power to the power amplifier, P.sub.out is the output power from the power amplifier, I.sub.C is the collector current flowing in the HBT, I.sub.B is the base current of the HBT, and I.sub.B1 is the base current in idle time. As shown in FIG. 6, it is desired that the base current I.sub.B1 in idle time is reduced and the base current in amplification time is increased.
FIG. 7 is a circuit diagram illustrating an HBT power amplifier 200 having an amplifier stage including an AlGaAs/GaAs HBT. The power amplifier 200 comprises a bias circuit 200a and an amplifier stage 200b. The bias circuit 200a includes HBTs Tr.sub.201 and Tr.sub.202, and the amplifier stage 200b includes an HBT Tr.sub.A. In FIG. 7, RF.sub.in denotes an input terminal of an RF (radio frequency) signal, RF.sub.out denotes an output terminal of the RF signal, V.sub.CC denotes a power supply voltage, V.sub.ref denotes a voltage for bias setting obtained by resistance division of the power supply voltage V.sub.CC, L.sub.1 denotes a matching inductor, C.sub.1 and C.sub.2 denote matching capacitors, L.sub.C denotes an RF choke inductor for RF separation between the bias circuit 200a and the amplifier stage 200b, I.sub.201 .about.I.sub.204 denote currents flowing in the bias circuit 200a, and I.sub.C denotes a collector current of the HBT Tr.sub.A.
FIG. 8 is a circuit diagram illustrating another HBT power amplifier 300 having a GaAs HBT in an amplifier stage. In FIG. 8, the same reference numerals and characters as those shown in FIG. 7 designate the same or corresponding parts. The power amplifier 300 comprises a bias circuit 300a and an amplifier stage 300b which is identical to the amplifier stage 200b shown in FIG. 7. The bias circuit 300a includes HBTs Tr.sub.301 to Tr.sub.306. In the bias circuit 300a, I.sub.301 and I.sub.303 .about.I.sub.308 denote currents, and R.sub.1 and R.sub.3 .about.R.sub.5 denote resistors.
In the power amplifiers 200 and 300 each having an AlGaAs/GaAs HBT Tr.sub.A in the amplifier stage, since GaAs series HBTs similar to the HBT Tr.sub.A are used in the bias circuit, the HBTs in the bias circuit and the HBT in the amplifier stage can be fabricated simultaneously, whereby the bias circuit and the amplifier stage are integrated on the same GaAs substrate.
In the HBT power amplifier 200 shown in FIG. 7, since the HBT Tr.sub.201 and the HBT Tr.sub.A are disposed between the bias setting voltage V.sub.ref of the bias circuit 200a and the emitter electrode of the HBT in the amplifier stage 200b, base-to-emitter voltages of these two HBTs are accumulated. Since an AlGaAs/GaAs HBT requires a base-emitter voltage V.sub.be of 1.4.about.1.5V in its ON state, the bias setting voltage V.sub.ref of the bias circuit 200a must be at least 2.8V (=2.times.1.4) for normal operation. Since the bias setting voltage V.sub.ref is generated from the power supply voltage V.sub.CC, the power supply voltage V.sub.CC must be at least 2.8V for normal operation.
Likewise, in the HBT power amplifier 300 shown in FIG. 8, since the HBT Tr.sub.301 and the HBT Tr.sub.A are disposed between the power supply voltage V.sub.CC for bias control in the bias circuit 300a, and the emitter electrode of the HBT in the amplifier stage 300b, base-to-emitter voltages of these two HBTs are accumulated. Since an AlGaAs/GaAs HBT requires a base-emitter voltage V.sub.be of 1.4.about.1.5V in its ON state, the power supply voltage V.sub.CC of the bias circuit 300a must be at least 2.8V (=2.times.1.4) for normal operation.
Generally, in a portable telephone system operating at a power supply voltage of about 3V using a lithium ion (Li.sup.+) battery, an NiCd battery or an NiMH battery, since the terminal voltages of these charging batteries are about 2.7V, the operating voltage of the power amplifier must be at least 2.7V. In the conventional power amplifier described above, however, because of the base-emitter voltage V.sub.be determined by the physical constant of the HBT, the bias circuit does not operate when the power supply voltage V.sub.CC is 2.7V.
As described above, in the conventional power amplifier, sufficient operation cannot be realized when the power supply voltage V.sub.CC is lower than 2.7V.